China's biggest ride-sharing company apologised on Thursday over the killing of a passenger, apparently by her driver, and said it had to win back the trust of users after the "tragedy".

The young woman passenger was killed in Zhengzhou city last week, compounding fears about passenger safety that have been plaguing the ride-sharing giant Didi Chuxing, which is valued at $50 billion.

Didi was "deeply saddened by and sorry about the tragedy", the company said in a statement, adding that it had apologised to the family of the 21-year-old flight attendant killed en route from the city's airport hotel to downtown.

"We need to step up to win the trust of our users. Our responsibilities in this case are undeniable," Didi said in a statement to Reuters.

Didi Chuxing Technology Co is the world’s largest ride-hailing firm by number of rides, thanks to its commanding market share in China, where it has 450 million users.

It completed more than 7.4 billion rides last year, not quite double Uber’s count.

Uber learned the hard way about Didi's brawn. After waging a campaign to crack the Chinese market, Uber in 2016 sold its operation to Didi in exchange for a 17.5 percent stake in the Chinese firm, which also made a $1 billion investment in Uber.

Didi said police were looking for the person suspected to have killed the woman and it would work closely with law enforcement authorities on the case.

Zhengzhou police did not comment on the killing but urged customers to exercise caution.

"Do not engage in disputes with drivers," police said in n advice posted on social media, adding: "be accompanied when taking rides at night".

The killing of the woman has been discussed extensively on social media platforms in China with many expressing fears for their safety.

"Didi really needs to step up internal supervision; who is going to ensure our safety?" said one user of the Weibo platform.

Didi is on a push to expand globally after sealing its dominance in China.

It has made a foray into other businesses including a recent cash-burning experiment in food delivery that pitted it against Meituan-Dianping, China's largest provider of on-demand online services.